President Barack Obama learned Thursday that he may have a wait on his hands before Congress signs off on his plan to train and equip Syrian rebels, a key plank in his strategy to destroy Islamic State radicals.

Obama oversaw Thursday the 13th anniversary commemorations of the September 11 attacks in 2001 -- which fell, in a trace of bitter irony, hours after he steeled Americans for a new battle against Muslim fanaticism in the Middle East.

At the same time, on Capitol Hill, the machinery of US government slowly slipped into gear, as lawmakers digested and considered how they would respond to Obama's plan to take on IS, which has overrun swaths of Iraq and Syria, beheading two American reporters in its brutal surge.

Several House Republicans emerged from a meeting to say a quick vote on authorization to empower vetted, moderate Syrian rebels was looking unlikely. Lawmakers are due to leave town in a week to campaign ahead of November's mid-term elections.

But Republican congresswoman Marsha Blackburn told reporters Congress may stay in town for another week.

"It's important to address this and it's important to get it right," she said.

House Speaker John Boehner said he personally backed Obama's plan, but that caucus members were worried that the broader Obama strategy was insufficient.

"If our goal is to eliminate ISIL (IS), there's a lot of doubt whether the plan that was outlined by the president last night is enough to accomplish that mission," Boehner told reporters.

"We'll make a decision sometime next week on how we will proceed."

Senior administration officials went to Capitol Hill to brief lawmakers in classified sessions on the plan, which also foresees new air strikes against IS in Syria, expanded attacks in Iraq and new support for Iraqi government forces.

Obama has asked Congress to approve $500 million to train Syrian rebel groups, a request he first tabled in May.

- No Fear -

As he remembered the attacks 13 years ago, with a moment's silence at the White House and a visit to the Pentagon, Obama may have had the new challenge from IS on his mind, condemning the Al-Qaeda hijackers that smashed planes into buildings in New York and the Pentagon.

"They sought to break our spirit and to prove to the world that their power to destroy was greater than our power to persevere and to build," Obama said.

But the president said Al-Qaeda had underestimated US endurance.

"We carry on because as Americans we do not give in to fear. Ever."

The White House meanwhile insisted that the president already had the authority to escalate the anti-IS operation in the Middle East, under a law known as the Authorization for Use of Military Force (AUMF), passed to endorse then president George W. Bush to take the battle to Al-Qaeda in 2001.

White House spokesman Josh Earnest argued that since IS emanated from Al-Qaeda, shared its aspirations to build a caliphate and was previously known as Al-Qaeda in Iraq, it was covered by the 2001 law.

Obama said in his televised address that he was ready to hit IS targets in Syria for the first time, to reconstitute Iraq's armed forces and to train and equip "moderate" Syrian rebel groups, and to lead a broad international coalition to take on IS.

"Our objective is clear: we will degrade, and ultimately destroy, ISIL through a comprehensive and sustained counter-terrorism strategy," Obama said, using an alternative acronym for the rebel group.

But he was also at pains to stress that his approach would differ sharply from the large land wars launched by Bush in the aftermath of the September 11 attacks and would look more like US counter-terror operations in Somalia and Yemen.

In effect, Obama is attempting to establish a framework for future presidents to wage war on terrorism in a way that does not drain US power or leave the country bogged down in hostile foreign lands.

- Mixed reaction -

Reaction to the president's speech was mixed, with prominent Republicans grudgingly backing his toughened approach, after the president admitted two weeks ago he did not have a strategy to combat the group in Syria.

But some key Washington players questioned the core goals of the president's strategy.

"He didn't say we are going to do whatever it takes to defeat them," said Republican Senator Marco Rubio, a possible 2016 presidential candidate.

"What if the Syrian rebels, no matter what we do, are not able to become a force that can confront them in Syria?" Rubio told CNBC.

"What if the Iraqi military forces don't come together fast enough?"

But an important Obama ally, Democratic Senate Majority leader Harry Reid, said Obama had delivered a "stirring" speech and accused some Republicans of taking "cheap shots" at the president.